Spin Hamiltonian for Even-Electron Systems Having Even Multiplicity

The form of spin Hamiltonian necessary to represent the most general zero-field splitting and lowest order magnetic and quadrupole interactions is investigated for systems having an even number of electrons and an even multiplicity. It is shown that, when referred to principal axes, two components of the $g$ tensor are necessarily identically zero. Various other peculiar features emerge. In particular, it is shown that in interpretations of electron spin resonance measurements using a spin Hamiltonian for a non-Kramers doublet, considerable misapprehension and error exist in the literature.